1. Field of the Invention
The invention relates generally to the field of seismic data acquisition and processing to image subsurface Earth formation structures. More specifically, the invention relates to methods for acquiring and processing passive seismic data, that is, seismic data acquired without the use of a controllable source.
2. Background Art
Passive seismic emission tomography is a process in which an array of seismic sensors is deployed in a selected pattern on the Earth's surface (or on the water bottom in marine surveys) and seismic energy is detected at the sensors that emanates from various seismic events occurring within the Earth's subsurface. Processing the signals detected by the sensors is used to determine, among other things, the position in the Earth's subsurface at which the various seismic events took place.
Applications for passive seismic emission tomography include, for example, determining the point of origin of microearthquakes caused by movement along geologic faults (breaks in rock layers or formations), movement of fluid in subsurface reservoirs, and monitoring of movement of proppant-filled fluid injected into subsurface reservoirs to increase the effective wellbore radius of wellbores drilled through hydrocarbon-producing subsurface Earth formations (“fracturing”). The latter application, known as “frac monitoring” is intended to enable the wellbore operator to determine, with respect to time, the direction and velocity at which the proppant filled fluid moves through particular subsurface Earth formations.
Passive seismic emission tomography for the above types of interpretation includes determining what are seismic-induced events from within the signals detected at each of the seismic sensors, and for each event detected at the seismic sensors, determining the spatial position and time of the origin of the seismic event. Passive seismic interpretation methods known in the art are undergoing continuous improvement to better resolve the source of seismic events originating from the Earth's subsurface. There continues to be a need for improved methods of passive seismic emission tomography.